DL are renowned for chopping and changing. Route did well in the summer, if it wasn't making money when it was full with pax and cargo then DL must be doing something wrong. Winter was a different story however it seems although I thought it would have been sustainable still even with the lower winter load factors.
They maybe should have tried a seasonal flight to ATL aswell as JFK. However then comes the problem of BAA and in their inability to provide adequate facilities when required. There would be nowhere to put 2 CO B752s, 1 DL B763 and 1 DL B752 if the above situation was to have become reality, but it hasn't so it's fairly irrelevant I suppose.
